This invention relates to a comparator circuit and, in particular, to a comparator circuit which is responsive to differential input signals ranging over the full operating potential (i.e. from rail to rail) and which is capable of producing output signals which can swing from rail to rail (i.e. over the full range of the operating potential applied to the comparator circuit).
A circuit designed to respond to differential input signals ranging over the full operating potential aplied to the circuit is shown, for example, in U.S. Pat. No. 4,377,789 titled "Operational Amplifier Employing Complementary Field-Effect Transistors" issued to M.V. Hoover. However, the output of the circuit in the Hoover (4,37,789) patent does not swing from rail-to-rail (i.e. from V.sub.DD to V.sub.SS). In fact, the output is limited to a value of V.sub.DD minus the threshhold voltage of a P-type transistor and to a value of V.sub.SS plus the threshhold voltage of an N-type transistor. The inability to produce output signals which swing all the way to V.sub.DD or V.sub.SS is problematic. For example, such an output can not drive a subsequent CMOS logic circuit properly, resulting in unwanted and undesirable power dissipation.
Another circuit designed to respond to differential input signals ranging from rail-to-rail is shown in U.S. Pat. No. 4,333,058 titled "Operational Amplifier Employing Complimentary Field-Effect Transistors" and also issued to M.V. Hoover. The U.S. Pat. No. 4,333,058 reference includes a first differential amplifier stage (10) comprised of differentially connected transistors (P1, P2) and N-type load transistors (N3, N4) driving a current sinking transistor (N5) and a second differential amplifier stage (30) comprised of differentially connected N-type transistors (N1, N2) and P-type load transistors (P3, P4) driving a current sourcing transistor (P5). The same differential input signals are applied to the two differential amplifier stages (10 and 30).
When the differential signals are within a P-threshhold voltage (V.sub.TP) of the most positive operating voltage (V+), the first differential amplifier stage (10) is inoperative in that both P1 and P2 are turned-off. Current sinking transistor N5 is then either off or in an indeterminate state. Concurrently, the second differential amplifier stage 30 is operative driving current sourcing transistor P5 as function of the differential input signal. However, since N5 is off or indeterminate, the output response of the amplifier is not a true or linear function of the differential input signal.
In a similar manner, when the differential input signals are within an N-threshhold voltage (V.sub.TN) of the lowest operating voltage (V-), the second differential amplifier stage (30) is inoperative in that both N1 and N2 are turned-off. Current sourcing transistor P5 is then off or in an indeterminate state. Concurrently, the first differential amplifier stage (10) is operative driving the current sinking transistor N5 as a function of the input signal. However, since P5 is off or indeterminate, the output response of the amplifier is not a true or linear function of the differential input signal.
The problems discussed above are overcome in circuits embodying the invention.